 Because blocks are normally not split between tracks, specification of an incorrect block size can waste up to half of each track.Most often, the key is omitted and the record is located sequentially or by direct cylinder-head-record addressing.If it is present, the key is typically a copy of the first bytes of the data record (for "unblocked" records, or a copy of the highest key in the block, for "blocked" records), but can be any data which will be used to find the record, usually using the Search Key Equal or Search Key High or Equal CCW.